Years ago, in an article on car cleaning, the best type of cleaner was a steam cleaner. BUT there were cautions to have somebody do it that knew what to NOT steam clean. Things like bearings which could be de-lubed by the steam, or similar. End result, a poor operator could do more damage than not, they noted, yet everything would be spotless when they were done.
If most of the deposits you're trying to remove are (or were, before they dried out) oil based, then putting an oil-based solution on them first, to soften them back up before you try to wash them off, is best. Why the diesel and Varsol solvent tend to work as good as they do. Like using car wax to soften "road tar" to remove it by first liquifying it somewhat.
Other than the truck/bus repair/wash shops, you might consider "graffiti removal" operations, who use a pressure washer situation. Which also means they are "portable" and come to where you are.
Only thing is that if you're looking to clean "every nook and crannie", then you'll probably end up removing some surface coatings/sealers in the process too. Which will have to be replaced somehow or another.
Just some thoughts,
CBODY67
